1. Field of the Invention
The present invention relates to the field of petroleum extraction and is concerned with an agent for propping fractures formed in the walls of an oil well which passes through geological formations.
2. The Prior Art
The method of bringing about stimulation of wells by fracturing of geological strata, for example, by injecting a fluid at high pressure at the level of the formation which it is desired to fracture, then by injecting solid granular materials (propping agent) in order to prevent the fractures from closing up.
Various materials have been proposed. However, in geological formations at great depths, whose use for extraction of oil or natural gas is being developed, the stresses exerted on the piles of solid grains are such that many materials which are usually used, especially siliceous sand or glass balls, become fragmented to a considerable extent. It is found that the fragments formed reduce the permeability of the fracture. Because of this, the production from fractured formations is decreasing.
It has moreover been observed that the behaviour of propping agents with respect to the stresses encountered in this type of operation is not only in relation to the resistance of individual grains to crushing. Hence supporting agents with a lesser breakage rate in a multilayer bed have been sought.
At the present time there are propping agents on the market which consist of sintered alumina or bauxite. Societe Europeenne des Produits Refractaires of Nevilly-Sur-Seine, France, is also marketing a propping agent which consists of zirconia and silica in proportions which are such that the ratio by weight ZrO.sub.2 /SiO.sub.2 is at least 1.5, alumina and sodium oxide can be likewise included in a optional manner (see French Pat. Nos. 2,306,327 and 2,359,274 and U.S. Pat. No. 4,072,193). All the propping agents with high mechanical resistance available on the market have high densities i.e., at least 3300 kg/m.sup.3 and up to 3800 kg/m.sup.3.
The injection of propping agents whose density is high creates transport difficulties at the bottom of the fractures owing to their rate of sedimentation in the fracturing fluid. It is hence necessary to use more viscous and more dense fluids in order to limit sedimentation as much as possible and to enable a progressive placing right to the bottom of the fractures formed. These fluids do not always give the best results (deterioration of the fluid, damage to the formation) and are, moreover, very costly.
It would hence be of interest on the technical and economical level to have available propping agents with high mechanical resistance for large depths, but which have a density which is less than the values encountered for the propping agents available at present. Besides the economic value offered by the utilisation of a less complex fracturing fluid, the lowering of the density of the propping agents will result in a decrease in the rate of sedimentation and thus will contribute to a better efficiency of the operation by promoting the placing of propping agents into a larger proportion of the fracture. In a fluid, the rate of sedimentation of a particular solid is proportional to the difference of the densities of the solid and of the carrying liquid. All else being otherwise equal, the reduction of the density of a propping agent from 3800 to 3200 kg/m.sup.3 will result in a reduction of the rate of sedimentation by 21% in a fluid with a density of 1000 kg/m.sup.3 and of 24% in a fluid with a density of 1300 kg/m.sup.3.
The object of the present invention is to supply a propping agent with a high mechanical resistance, especially in the multilayer bed, with a density less than 3250 kg/m.sup.3.